The present invention relates to a process for recovering energy and chemicals from spent liquor obtained in pulp production and to a reactor for carrying out this process.
Burning fossil fuels such as coal, oil and natural gas constitutes an alternative to nuclear power for producing heat. The alternatives also include biomass fuels, in particular the combustion of spent liquors obtained from the cellulose industry. Two objectives are aimed at when burning spent liquors from the sulphate pulp industry, the first being that the organic wood substance is combusted so that its heat of combustion is converted to useful thermal energy, and the second being that the inorganic chemicals in the spent liquor are recovered and converted to active form. Under-stoichiometric condition is required when the sulphur is to be recovered in sulphide form, while at the same time the recovery of energy requires over-stoichiometric condition. Two opposing processes are thus required to take place simultaneously within a common space, which results in optimization problems when using traditional soda recovery boiler technique. Hitherto one has tried to solve the problem by using various process levels in the soda recovery boiler in which under-stoichiometric condition prevail in the bottom and over-stoichiometric condition in the upper part. Optimization problems of the soda recovery boiler include inter alia that the chemicals recovered from e.g. sulphate pulp production contain a certain amount of oxidized sulphur in the form of sodium sulphate, sodium sulphite and sodium thiosulphate. This is due to difficulties in maintaining controlled conditions in the bottom region of the boiler. Besides this, a certain amount of dust is also emitted from the bottom region of the boiler. The dust contains sodium and sulphur which are oxidized higher up in the soda recovery boiler, forming sodium sulphate which is separated out and returned to the chemical cycle. The oxidized sulphur is chemically inactive in the pulping process and thus constitutes a ballast in the chemical cycle.
U.S. Pat. No. 4,808,264 (corresponding to SE 448 173) describes a process constituting an alternative to the soda recovery boiler process and based on a gasification technique. The organic substances of the black liquor are gasified in a first step to produce substantially CO, CO.sub.2, H.sub.2 and H.sub.2 S in a pressurized reactor by means of so called "flash-pyrolysis", whereby the remainder obtained consists substantially of the inorganic components of the spent liquor in solid or molten form. In such a gasification of organic material a portion of residual carbon is normally obtained, the quantity of residual carbon being dependent upon several factors such as the reaction temperature, air/fuel-ratio and fuel injection technique. The gas obtained from gasification is conveyed further for cleaning and can then be used as fuel. Since the gasification is carried out during addition of oxidation air, nitrogen is also supplied. Nitrogen constitutes ballast and the amount of oxidation air should therefore be minimized. However, if gasification is carried out at under-stoichiometric conditions a carbon residue ("char") is very easily formed. Technically, the design of the apparatus and process for the known reactor is essential in particular with respect to thermodynamic conditions such as reaction time, temperature, turbulence and material atomization so that as large a reaction surface as possible is formed. The low air/fuel-ratio also causes particular difficulties in maintaining high turbulence in the reaction zone as compared with a complete combustion process. Flow conditions with formation of laminar films of oxidation air around the particles easily occur and optimization of the parameters mentioned above becomes a very difficult task. At higher pressure in the gasification chamber the density of the gas increases, thus causing further limitations of the turbulence in the gasification chamber. Temperature and the oxidation or reaction processes will therefore vary.
The conditions for optimizing a combustion process and those for optimizing a gasification process are thus significantly different. This is particularly so when the gasification process comprises several steps, i.e. chemical recovery which is very difficult to optimize, besides the actual gasification of organic material.
SE 458 799 describes combustion of fluid fuels dispersed in combustion air and subjected to low frequency sound. Combustion of fuel occurs necessarily in excess of air and thus differs essentially from a thermal decomposition process in which the material is thermally decomposed with a limited supply of air (gasification) or without supply of air (pyrolysis). This patent specification neither reveals nor suggests the use of low frequency sound in such a substantially different technique as the recovery of energy and chemicals by means of such a substantially different process as an endothermal decomposition process with its specific optimization problems.